The skin is the largest organ in the body and plays critical roles in sensation, thermoregulation and protection from the environment. The outermost layer of the skin, the epidermis, is essential for animal survival, acting as a barrier to keep out toxins and microbes while preventing excessive fluid loss. The appropriate balance between cell division and differentiation is crucial for epidermis formation and maintenance. Significant advances have been made in deciphering the mechanisms of epidermis formation, homeostasis and wound response, however our understanding of the factors that control the proliferation and differentiation of epidermal keratinocytes is incomplete. This proposal focuses on the analysis of a novel mouse mutant, shorthand (shd) that displays abnormal epidermal differentiation, with the long-term objective of understanding how the affected gene regulates epidermal integrity during embryonic development and disease. There are several key steps in epidermis formation including a) the transition from a monolayer to a stratified epithelium, b) the switch from proliferation to differentiation, and c) the formation of the epidermal barrier. We will use the shd mutant, to investigate the role of the shd gene in each of these steps during epidermal development. The Specific Aims of this proposal are to 1) define the role of shd in epidermal development, 2) determine the function of the shd gene and 3) better define the relationship between shd and known keratinocyte differentiation genes. Experiments in Aim 1 will examine the role of shd at distinct stages of epidermal development. These experiments will test how shd regulates stratification of the epidermis, keratinocyte proliferation and differentiation. Experiments in Aim 2 will focus on determining when and where the shd gene acts during epidermal development. Determining requirement for this key keratinocyte differentiation gene will broaden our understanding of the mechanisms of epidermal regulation. Experiments in Aim 3 will examine genetics and molecular interactions between shd and known epidermal regulators. These experiments will help define the architecture of the epidermal keratinocyte differentiation pathway. Defects in the regulation of epidermal proliferation and differentiation can lead to developmental defects and cancer. For example, Ikk1 and stratifin, appear to have similar developmental roles as shd and have been implicated in tumorigenesis. Thus, identifying the shd gene and defining its role in epidermis formation and differentiation should provide fundamental insights into animal and human development and provide necessary background for understanding and treating human disease.